The Owenkillew River rises in Davagh Forest, flowing swiftly downslope towards Newtownstewart, some 40 km to the west. The upper section is typical of fast-flowing spate rivers that are ultra-oligotrophic (full of lots of oxygen but no nutrients for plant growth). A number of aquatic mosses such as Fontinalis squamosa, Rhynchostegium riparioides and Hygrohypnum luridum, occur in the deep channel and are typical species of upland streams.

Moving downstream, the river channel is wider and more diverse whilst maintaining its nutrient poor status. In the mid-reaches, trees frequently line the river channel. Along these stretches, vascular plants are dominant and include stands of stream water-crowfoot and yellow water-lily in addition to some aquatic mosses. Further downstream, the river banks support stands of remnant woodland with its own distinct flora and fauna.

The Owenkillew River is of particular importance for the freshwater Pearl Mussel, supporting the largest known population in Northern Ireland, where it is one of the few rivers to still retain a significant and viable population of this rare shellfish. In addition, both salmon and brown trout are present in the river, in addition to species such as Brook lamprey. Otters, dipper and Kingfishers are present along the length of the river.